The present invention relates to a tri-color lamp. The lamp comprises a mixture of two phosphors and a light emitting diode for an excitation energy source. In particular, the lamp employs a blue LED and a mixture of red and green phosphors for the production of white light.
There is an ongoing need to generate new phosphor compositions to improve efficiency and color quality in luminescent devices, particularly in the production of white light. Phosphors are luminescent materials that can absorb an excitation energy (usually radiation energy) and store this energy for a period of time. The stored energy is then emitted as radiation of a different energy than the initial excitation energy. For example, xe2x80x9cdown-conversionxe2x80x9d refers to a situation where the emitted radiation has less quantum energy than the initial excitation radiation. Thus, the energy wavelength effectively increases, and this increase is termed a xe2x80x9cStokes shift.xe2x80x9d xe2x80x9cUp-conversionxe2x80x9d refers to a situation where the emitted radiation has greater quantum energy than the excitation radiation (xe2x80x9cAnti-Stokes shiftxe2x80x9d).
Improvements in efficiency and color quality in phosphor-based devices are constantly being developed. xe2x80x9cEfficiencyxe2x80x9d relates to a fraction of photons emitted with respect to a number of photons initially provided as excitation energy. Inefficient conversion results when at least a portion of the energy is consumed by non-radiative processes. Color xe2x80x9cqualityxe2x80x9d can be measured by a number of different rating systems. xe2x80x9cChromaticityxe2x80x9d defines color by hue and saturation. xe2x80x9cCIExe2x80x9d is a chromaticity coordinate system developed by Commission Internationale de l""Eclairage (international commission on illumination). The CIE Chromaticity Coordinates are coordinates that define a color in xe2x80x9c1931 CIExe2x80x9d color space. These coordinates are defined as x, y, z and are ratios of the three standard primary colors, X, Y, Z (tristimulus values), in relation to the sum of the three tristimulus values. A CIE chart contains a plot of the x, y and z ratios of the tristimulus values versus their sum. In the situation where the reduced coordinates x, y, z add to 1, typically, a two-dimensional CIE (x, y) plot is used.
White-like colors can be described by a xe2x80x9ccorrelated color temperaturexe2x80x9d (CCT). For example, when a metal is heated, a resulting light is emitted which initially glows as a red color. As the metal is heated to increasingly higher temperatures, the emitted light shifts to higher quantum energies, beginning with reddish light and shifting to white light and ultimately to a bluish-white light. A system was developed to determine these color changes on a standard object known as a blackbody radiator. Depending on the temperature, the blackbody radiator will emit white-like radiation. The color of this white-like radiation can then be described in the CIE chromaticity chart. Thus, the correlated color temperature of a light source to be evaluated is the temperature at which the blackbody radiator produces the chromaticity most similar to that of the light source. Color temperature and CCT are expressed in degrees Kelvin.
A xe2x80x9ccolor rendering indexxe2x80x9d (CRI) is established by a visual experiment. The correlated color temperature of a light source to be evaluated is determined. Then eight standard color samples are illuminated first by the light source and then by a light from a blackbody having the same color temperature. If a standard color sample does not change color, then the light source has a theoretically perfect special CRI value of 100. A general color rendering index is termed xe2x80x9cRaxe2x80x9d, which is an average of the CRIs of all eight standard color samples.
Older white lamps involved emission of light over a broad wavelength range. It was then discovered that a white-like color can be simulated by a mixture of two or three different light colors, where each emission comprised a relatively narrow wavelength range. These lamps afforded more control to manipulate the white color because emissive properties (emission energy and intensity) of the individual red, green and blue light sources can be individually tailored. This method thus provided the possibility of achieving improved color rendering properties.
An example of a two-color lamp comprises one phosphor and an excitation energy source. Light emitted by the phosphor combines with unabsorbed light from the excitation source to produce a white-like color. Further improvements in fluorescent lamps involved three different light colors (i.e. a tri-color lamp) resulting in white light at higher efficiencies. One example of a tri-color lamp involved blue, red and green light-emitting phosphors. Other previous tri-color lamps comprised a combination of light from two phosphors (a green and red phosphor) and unabsorbed light from a mercury plasma excitation source.
Previous tri-color lamps involving a mercury plasma excitation source, however, suffer many disadvantages including: (1) a need for high voltages which can result in gaseous discharge with energetic ions; (2) emission of high energy UV quanta; and (3) correspondingly low lifetimes. Thus, there is an ongoing need for devices that overcome these deficiencies.
Finally there remains a continued challenge to uncover phosphor compositions and mixtures of these compositions to provide improved properties, including improved efficiency, color rendering (e.g. as measured by high color rendering indices) and luminance (intensity), particularly in a tri-color, white lamp.
One aspect of the present invention provides a composition comprising a mixture of a first phosphor and a second phosphor. Each phosphor comprises a host sulfide material and a rare earth dopant and each phosphor is capable of being excited by a common light emitting diode.
Another aspect of the present invention provides a composition comprising a mixture of a first phosphor and a second phosphor. Each phosphor comprises a host material and a rare earth dopant. The first phosphor is capable of being excited by a light emitting diode and the second phosphor is capable of being excited by an emission of the first phosphor.
Another aspect of the present invention provides a device comprising a light emitting diode, for emitting a pattern of light. The device further comprises a composition comprising a mixture of a first phosphor and a second phosphor. Each phosphor comprises a host sulfide material and a rare earth dopant and the composition is positioned in the light pattern.
Another aspect of the present invention provides a device comprising a green phosphor and a blue light emitting diode, for providing an excitation radiation to the phosphor.